Ultrasonic testing systems have been designed and built to meet the needs of a variety of applications. One application is non-destructive evaluation, where ultrasonic acoustic energy is applied to an object-being-probed (a "specimen"), and the echo of reflected or scattered acoustic energy, caused by cracks or density differentials, is received and analyzed to reveal the internal and/or surface structure of the specimen.
A typical ultrasonic transducer is a quartz crystal or other piezo-electric device which can convert a high-frequency (i.e. &gt;20,000 Hz) alternating-current electrical signal into a corresponding acoustical signal and vice versa. The transducer is often modeled as a tuned LC (inductance-capacitance) circuit, with one resonant frequency. Real-world transducers can have locally-resonant characteristics at a multiplicity of frequencies. At a resonant frequency, a greater amount of acoustical energy is generated from a given amount of electrical energy (and vice versa) than at other non-resonant frequencies. Any input electrical signal which is not converted into acoustical energy is typically converted into waste heat in the transducer.
One technique for broadening the bandwidth of the resonant frequency is to provide mechanical damping for the transducer.
Acoustic energy is typically coupled from the transducer to the specimen by a coupling medium, often a liquid such as water or oil. The coupling medium is designed to minimize acoustic discontinuities in the path of the acoustic wave which would otherwise lessen the energy transmitted between the transducer and the specimen. Once inside the specimen, the acoustic wave reflects and scatters from cracks and other acoustic transmission discontinuities in the specimen. The acoustic signal thus echoed by the internal features of the specimen is then received by a transducer. If the same transducer is used for both transmission and reception of the acoustic signal, then the system is called a "pitch-catch mode" system; if separate transducers are used for transmission and reception, then the system is called a "pulse-echo mode" system. The received signal is converted back into an electrical signal and then amplified, analyzed, and displayed.
The wavelength of an acoustic wave at a given frequency is a function of the velocity of the wave in the transmission medium.
One type of ultrasonic testing system is the "continuous wave" system. A single-frequency, sine wave (or approximately sine wave) electrical signal at or near the resonant frequency of the transducer is coupled to the transducer, which converts the electrical signal into a corresponding acoustic sine wave. This acoustic wave is coupled to the specimen, and the echo received (typically by a separate transducer) and amplified (typically by a tuned amplifier). The amplitude and phase of the echoed signal is then analyzed. This technique is often used to measure velocities of physical components internal to the specimen (e.g., blood velocity in a vein), or attenuation of the signal due to inhomogeneities. This type of system is simple, relatively inexpensive, and can do quite accurate measurement of velocities using resonance techniques; however, since range information is not available, it is difficult to pinpoint an internal flaw region.
Another type of ultrasonic testing system is the "continuous-wave, swept-frequency" system. A ramp generator drives a variable-frequency oscillator to generate the swept-frequency transmission signal which drives the transmitting transducer. The same ramp generator tunes a variable-frequency tuned amplifier which amplifies the received signal (which is typically received by a separate transducer), which is then analyzed and displayed. This type of system has greater frequency diversity, and can do automated measurements over a range of frequencies; however, since range information is still not available, it is difficult to pinpoint an internal flaw region, and expensive components and broadband transducers are required.
Another type of ultrasonic testing system is the "pulsed single-frequency" system. A single-frequency oscillator is amplitude-modulated with a pulse; the resulting "tone burst" drives the transducer with a few cycles (e.g., ten cycles) of sine wave. Because the tone burst has a beginning and end, it is possible to measure time delay, as well as amplitude and phase information; this allows measurement of depth in the specimen.
Another type of ultrasonic testing system is the "pulsed, swept-frequency" system. A ramp generator drives a variable-frequency oscillator to generate a slowly swept frequency transmission signal, which is amplitude-modulated with a pulse; the resulting "tone burst" drives the transducer with a few cycles (e.g., ten cycles) of sine wave whose frequency continuously varies. The received signal is then amplified, analyzed, and displayed. This type of system has greater frequency diversity, and because the tone burst has a beginning and end, it is also possible to measure time delay as well as amplitude and phase information; this allows measurement of depth in the specimen; however, acquisition of spectra and signals takes a longer time than with other systems, the system is complex, and expensive components and broadband transducers are required.
Yet another type of ultrasonic testing system is the "pulsed, broadband analog" system. Some systems use a single, high-voltage (approximately 100 to 300 volts) pulse with a wide spectrum of frequencies to drive the transducer. Other researchers have suggested that a step-function driver be used rather than the pulse-function driver. The reflected signal is received and amplified. This type of system has good frequency diversity, and because the pulse has a beginning and end, it is also possible to measure time delay as well as amplitude information; this allows measurement of depth in the specimen; however, phase information is difficult to extract, the system is complex, and expensive hazard-reduction precautions may be required for the high-voltage pulse. Since the transducer acts like a tuned L-C circuit, much of the energy of frequency components outside the resonant frequencies of the transducer goes into waste heat.
None of the above systems provide particularly efficient conversion of electrical energy into acoustical energy (and vice versa) combined with the ability to measure time delay. Some methods involve driving the transducer with voltage signals which can be dangerous in a medical environment. Other methods use a continuous-wave signal which is not very useful in echo-location of interior structures of the item being investigated. What is needed is a method and apparatus which maximize signal conversion, minimize voltages to the transducer, and facilitate measurement of phase shift, time delay, and signal attenuation in the specimen.